The present invention relates to the biotechnology field, in particular with proteomics. Proteomics is defined as a group of tools, techniques and methods very close related to the proteomes studies. The term proteome is used to define the protein complement of the genome.
Nowadays the combination of separation technologies with mass spectrometry and automatic database search has made possible the high-trough put identification of proteins in complex mixtures.
Most of the emerging techniques analyze the peptides generated by hydrolysis of the proteins by the combination of liquid chromatography and mass spectrometry.
In 1999 Link et al. (Link, A. J. et al. Direct analysis of protein complexes using mass spectrometry. Nat. Biotechnol. 17, 676-682, 1999), developed a method based in two dimensional liquid chromatography and mass spectrometry (LC-MS/MS). For this purpose they packed a microcapillary column with ion exchange and reverse phase media. By this way, all proteolytic peptides are initially absorbed by the ion exchanger. Then, a single fraction of peptides is transferred to the reverse phase using a discontinuing salt gradient. Finally, the peptides are directly eluted from the reverse phase to the mass spectrometer using an increased gradient of acetonitrile. The described procedure is repeated several times using increased salts concentration in order to released additional fractions of peptides from the ion exchanger. This method is better known as MudPiT (Multidimensional Protein Identification Technology). Using the MudPiT, Washburn M. P. et al. identified 1484 proteins from the yeast Saccharomyces cerevisiae (Washburn M. P. et al. Large-scale analysis of the yeast proteome by multidimensional protein identification technology, Nature Biotechnology 19, 242-247, 2001). Although the MudPiT dramatically accelerated the proteins identification procedure, the relative quantitation of proteins could not be easily achieved. In this sense, Washburn M P et al., (Analysis of quantitative proteomic data generated via multidimensional protein identification technology. Analytical Chemistry. 74:1650-1656, 2002), quantified the proteins by the metabolic labeling of two Saccharomyces cerevisiae cultures. Cells were grown in enriched nitrogen-14/15 (14N/15N) media in a similar way to the previous studies of Oda et. al. (Accurate quantitation of protein expression and site-specific phosphorylation. Proc. Natl. Acad. Sci. USA 96, 6591-6596, 1999). These authors identified the proteins using two dimensional electrophoresis while Washburn M P et al. used MudPiT. For both cases the relative quantitation of proteins was done taking into account the relative intensities of the mass spectrum signals of a given labeled peptide according to the conditions cultures of its original sample.
Due to the high cost of reagents and media needed, the metabolic labeling is mainly applied to organisms such as bacteria and yeast. Moreover, in this kind of labeling all the nitrogen atoms of the proteins are labeled: the nitrogens atoms of the peptide bond as well as those of the amino acids side chains, which make impossible to predict the mass difference of homolog labeled peptides without the knowledge of the amino acid sequence.
An important step was taken by a group of authors using a different kind of metabolic labeling. In this case labeled essential amino acids are introduced to the cells cultures and hence they are incorporated to every expressed protein. This strategy was named SILAC (stable isotope labeling by amino acids in cell culture) and its potentialities have been proved using labeled amino such as leucine (1H/2D) and lysine (12C/13C) (S. E. Ong, B. Blagoev, I. Kratchmarova, D. B. Kristensen, H. Steen, A. Pandey, M. Mann, Mol. Cell Proteomics 1, 2002, 376-386); (Berger S J, Lee S W, Anderson G A, Lijiana P T, Tolić N, Shen Y, Zhao R, Smith R D, 2002, High-throughput Global Peptide Proteomic Analysis by Combining Stable Isotope Amino Acid Labeling and Data-Dependent Multiplexed-MS/MS. Analytical Chemistry 74:4994-5000); and (Precise Peptide Sequencing and Protein Quantification in the Human Proteome Through In Vivo Lysine-Specific Mass Tagging, J Am Soc Mass Spectrom 2003, 14, 1-7).
The enzymatic labeling of peptides has also been suggested and employed during the proteolytic digestion of the protein mixtures in the presence of water and oxygen-18 enriched water (H218O). In the latest case, one or two atoms of oxygen-18 are incorporated to the carboxy terminus of generated peptides. The comparative proteomic study is conducted mixing the samples of labeled and non-labeled peptides and analyzing by mass spectrometry, the pairs of homolog peptides. The ratio between the areas of the signals of a given peptide is proportional to the concentration ratio of the corresponding protein in the analyzed samples.
With this labeling technique there is not enough separation between the mass signals to avoid the overlapping of the isotopic envelops. Additionally, the incorporation of one or two 18O atoms produces a complex pattern which makes difficult the analysis. In this sense Yao et al. (Yao X, Afonso C, Fenselau C. Dissection of proteolytic 18O labeling: endoprotease-catalyzed 16O-to-18O exchange of truncated peptide substrates. J Proteome Res. 2003, 2,147-52) proposed a procedure to catalyze the complete incorporation of two 18O atoms to the C-terminus of peptides.
Besides the additional steps and experimental procedures, these methods are not saved from the possible appearance of other peptide(s) sharing part of the mass range covered by the isotopic envelope.
The inverse labeling methodology proposes a scheme to accentuate and emphasizes those ion signals that reflect the differential expression of proteins, by means of two experiments executed in parallel where each one use an inverse labeled respect to the other. The subtraction of the two mass spectra allows focusing the attention on those mass-changing-signals in one experiment respect to the other. The inverse labeling offers several advantages such as the identification of proteins which extreme expression changes and the detection of post-translational modifications. Furthermore, this methodology significantly reduces the time and efforts dedicated to the analysis of peptides MS/MS as well as protein identification and quantification. However, subtle changes in protein expression may no be detected by visual examination, especially when the differences do not exceed the noise of the mass spectra. In addition, the filtering and/or smoothing of the spectra results in lost of resolution and of relevant information in region of low signal-to-noise ratio. Another disadvantage of the inverse labeling method is the need of two experiments which certainly reduce the sensitivity.
Taking in to account the current resolution power of liquid chromatography and mass spectrometry systems, the analysis of all peptides generated during the hydrolysis of a complex mixture of protein results impracticable. For this reason, other alternatives of quantitative analysis of proteomes have emerged. In the new methods, the identification and quantification of protein is achieved by the selective isolation and analysis of a reduced group of peptides per protein present in the mixture.
An example of the emerging alternatives is the ICAT (isotope-code affinity tags) methodology (Quantitative analysis of complex protein mixtures using isotope-coded affinity tags. Gygi, S. P., Rist, B., Gerber, S. A., Turecek, F., Gelb, M. H., and Aebersold, R. Nat. Biotechnol. 17, 994-999, 1999). With ICAT, only cysteine containing peptides are isolated and analyzed through the use of reagent of three functional elements: an specific chemical reactivity, an isotopically coded linker and an affinity tag. In this method the free thiol of cysteines of a protein sample representative of a given cellular stage are modified with a light isotopically version of the ICAT reagent, while the sample representative of a second cellular stage is modified with the heavy version of the ICAT reagent. The two samples are combined, enzymatically digested and the cysteine contained peptides are isolated by affinity chromatography and analyzed by μLC-MS/MS.
The differential protein expression is quantify by measuring the relative intensities of the mass signals of the paired of peptides with identical sequences, but labeled with light and heavy isotopic version of the ICAT reagent.
An algorithm which reconstructs the spectra by means of a smoothing filter is used to determine the ratio of intensities. By this way the location and intensity of the local maxims (peaks) are identified. Finally, the average of the intensities of all identified peptides is determined to each protein.
The following limitations have been ascribed on the ICAT methodology:                proteins not containing cysteines residues are excluded from the analysis.        the size of the ICAT reagent causes interferences during the ionization and the mass spectra interpretation.        peptides labeled with the light and heavy versions of the ICAT reagent (H8/D8) could eluted significantly separated on RP-HPLC systems, a fact that could mislead the quantification process.        the quantification process could not be applied to labeling techniques that do not provide enough mass separation to avoid the overlapping of the isotopic distributions.        the quantification process could not offer reliable results if there is an overlapping of the isotopic distributions of a given peptide with the isotopic distributions of any of the pair (light/heavy) of homolog labeled peptides.        
In spite of the limitations described for this method, it continues being necessary to identify and determine relative levels of proteins expressions in complex mixtures, through the selective and specific isolation of a small group of peptide per protein, i.e. the previous simplification of the mixture of proteolytic peptides before the mass spectrometry analysis. The reduction of sample complexity would allow the identification of proteins poorly represented in the mixture, and would avoid the sequencing of many peptides from the same protein. Additionally, it is also necessary the development of methods to analyze and process the spectra of overlapped mass signals without complicating the experimental procedures.